The present invention is generally in the field of compositions for forming molding articles, and more particularly to compositions including powdered forms of glass, ceramic, metals, and thermoplastics.
A variety of useful molded products having complex shapes and useful mechanical strengths can be made from powdered forms of ceramics, metals, metal oxides, thermoset resins, high melt temperature thermoplastics, and combinations thereof. Examples of these products include aerospace components, biomedical implants, bonded diamond abrasives, cutting tools, engine and other mechanical parts, nozzles subject to continuous contact with abrasives, electronic devices, and superconductors. Forming techniques, such as slip casting, tape casting, extrusion, injection molding, dry pressing and screen printing, generally require the presence of a binder formulation that is mixed with the metal, ceramic, or mixed powder feed. The binder is a temporary vehicle for homogeneously packing the powder into the desired shape and then for holding the particles in that shape until the beginning of sintering (German, "Powder Injection Molding," (Metal Powder Industries Federation, Princeton, N.J. 1990); German and Bose, "Injection Molding of Metals and Ceramics," (Metal Powder Industries Federation, Princeton, N.J. 1997)). Sintering, or fusing, of the powder components is needed, for example, to obtain physical properties for the finished item that are suitable for the conditions of its end use. Sintering is an important process for thermoplastic resins, such as polyimides and fluoropolymers, that do not have a well-defined melt phase (Strong, "Plastics: Materials and Processing," (Prentice-Hall, Inc., Englewood Cliffs, N.J. 1996)).
One disadvantage with using traditional binders in the shape formation is that the molded product's physical properties and performance can be impaired by residual amounts of binder or binder decomposition products, by uneven removal of binder or binder decomposition products, or by voids formed by removal of binder or binder decomposition products. (Residual binder is not a problem in the limited circumstances when it is desirable to incorporate binder components into the final form by chemical or interatomic attraction.) Many products made from ceramic powders, metal powders, and blends thereof are used in applications where they are exposed to repeated stresses. Examples of these products include combustion engine parts, valves, rotors, and gear assemblies. Inclusion bodies derived from inadequate removal of binder, or voids resulting from combustion gases during removal, can facilitate cracking and failure of the parts in service. Electrical conductivity is another important performance requirement, for example in electronic parts such as printed circuit boards and superconductors, that can be adversely affected by inadequate binder removal or void formation caused thereby. Therefore, removal of the binder used in shape formation is generally a crucial step in the powder processing technique.
Techniques for the removal of undesirable binders include (1) thermal evaporation; (2) thermal decomposition; (3) chemical transformation to forms useful in the end product; (4) solvent extraction; (5) supercritical extraction; (6) diffusion and absorption of binder constituents to an absorbing surface surrounding the shape or wicking; and (7) depolymerization by thermal means, catalytic means, or a combination thereof. Removal of the binder usually is the slowest step in the powder injection molding process (German, "Sintering Theory and Practice," (John Wiley & Sons, N.Y. 1996); German and Bose, "Injection Molding of Metals and Ceramics," (Metal Powder Industries Federation, Princeton, N.J. 1997)). One binding system investigated for providing more rapid removal involves using polyacetals, particularly with injection molding processing, for example, as described in U.S. Pat. No. 5,155,158 to Kim and in WO 91/08993. The use of polyalkylene carbonates for use in such applications is disclosed in European Patent Application EP 0,463,769 A2. In theory, the polyacetal binders "unzip" or depolymerize, releasing formaldehyde, when exposed to nitric acid fumes in an incubator. Similarly, the polyalkylene carbonate binders "unzip" upon reaching a certain decomposition temperature, typically around 200.degree. C. Unfortunately, the use of nitric acid or other oxidants restricts the use of the polyacetals to those powders which are not susceptible to undesirable oxidation.
Other binder materials include polyoxalate and polymalonate polymers, which also are useful as rheological control agents in paste formulations, as described in U.S. Pat. No. 5,412,062 to Power et al. Polyalkylene carbonates, however, exhibit viscosity behavior that makes flow of the unformed metal/binder, ceramic/binder, or metal/ceramic/binder difficult.
Many of the characteristics of materials and compositions useful as binders are described in Shanefleld, "Organic Additives and Ceramic Processing," (Kluwer Academic Publishers, Boston 1996). Desirable features include (1) easy burnout, (2) strong adhesion to the powder and good cohesive strength, (3) solubility in fluidizing liquid, and (4) low cost. The binder material must be suitable for a variety of process conditions, since, for example, many powders must avoid exposure to air or water, or may require exposure to reducing gases or vacuum conditions, during processing.
It is therefore an object of this invention to provide molding compositions having improved binder removal characteristics.
It is another object of this invention to provide molding compositions suitable for use in a wide range of processing conditions.